# 7. sizing off grid.pdf

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• 8/20/2019 7. sizing off grid.pdf

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Wp

PV system sizing

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1. Site survey (structures, electrical consumers, meteorlogical data)

2. Determine consumer energy demand3. System voltage (12,24,48, 230V ?) 4. Determine PV array power

. eterm ne attery capac ty an vo tage 6. Determine charge controller voltage and

current

7. Determine inverter power 8. Determine cable size

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System voltage, depends on:

- Electrical devices (ac or dc) - Energy demand (approx. up to

1KVAh – 12V, up to 2,5KVAh – 24V...)

- Cable lengths PV – battery and battery – el. consumer

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Consumer energy demand: - Real power rating of ac devices (apparent

power) - Efficiency of inverter(app. 85-95%)

- . of rated power)

- Running time of devices (incl. Inverter)

= Battery discharge in Wh

divide by System Voltage

= Battery discharge in Ah

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Consumer Power demand: Device Wattage = inverter output power (VA)

cos phi

Inverter output = inverter input power (VA) Inv. eff.

-

Inverter

Output (VA)Input (VA)

Standby

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Consumer Energy demand: inverter input x hours = energy demand devices

inverter own consumption x hours = inverter energy demand

energy demand devices + inverter energy demand

=

consumer energy demand (VAh) = Battery discharge (Ah) Battery Voltage

Battery discharge (Ah) = Battery Capacity

DoD

Battery sizing:

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Battery discharge

Battery char e

C (Ah)

(Wh / Ah)

(Wh / Ah)

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Consumer

energy PV energy demand

Hours ?

Power ?

Cos Phi ?

Standby?

Eff.?

Out of MPP

operation losses

25%

P   e r f    o r

Charging

losses 5%

= Battery Discharge

(Wh)

= Battery Charge (Wh)

Electric losses 5%

m a n  c  e

6      5     %

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100Wh 65WhPV energy demand Consumer energy demand

Losses 35%

100Wh x 65% = 65Wh

65Wh / 65% = 100Wh

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Performance ratio (PR)

95%

65%

en ca cu a ng • Current increases slightly

with temperature and at

operating point (app. 5%) • losses for dust • battery losses • charge controller losses

• wiring losses

en ca cu a ng • operating voltage (13,5V)

is app 25% less than Vmpp

• losses for dust • battery losses • charge controller losses • wiring losses

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PV energy demand , Battery charge

= Battery discharge PerformanceRatio

PV Power , PV Current

= PV energy demand Peak sun hours

Choose the right module and quantity

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Battery sizing:

consumer energy demand (VAh) = Battery discharge (Ah) Battery Voltage

Battery discharge (Ah) = Battery Capacity

o

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Battery capacity depends on:

• daily battery discharge (Ah)

• Autonomy days

battery discharge x Autonomy days DoDC =

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Charge controller size depends on:

• current from PV module(s)

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Inverter size depends on:

• peak load of ac devices

Add 100% for CFL due to harmonics Consider starting current of motors

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cable size depends on:

• desired max voltage drop • length

• current

Allow max 3% voltage drop from PV to battery and from battery to consumer